1. Technical Field
The present invention relates to a wavelength splitting element, a method for manufacturing thereof and an optical module. Particularly, it relates to improvement of characteristics of a wavelength splitting element, improvement of a method for manufacturing a wavelength splitting element and an optical module using an improved wavelength splitting element.
2. Related Art
Recently, high-density wavelength division multiplexing such suitable for large capacity communication such as a wavelength-division multiplexing (WDM) method has been increased to address an increased communication demand. Data transmission capacity in the WDM method for multiplexing light signals with different wavelengths into a single optical fiber is determined based on the data transmission rate per wavelength and the number of wavelengths to multiplex.
JP-A-2005-164971 discloses a wavelength splitting element used in the WDM method and an optical module. FIG. 10 shows a structure of a wavelength splitting element 50 having two filters 51 and 52. The filters 51 and 52 includes filter films 51b and 52b, respectively, that form multilayer films made of SiO2, TiO2 or the like on one side of respective light transmission members 51a and 52a made of quartz, glass or the like. The filters 51 and 52 are arranged such that the normal direction of the filter film 51b is inclined at 45 degrees relative to X-axis of the coordinate axes (as shown at the right bottom in the drawing), the normal direction of the filter film 52b is inclined at an angle θ2 (6 degrees) and the filter films 51b and 52b face each other. The filter film 51b has wavelength characteristics, when a multiplexed light beam (in the wavelength bands λ1, λ2 and λ3) is incident at an angle of 45 degrees, to pass a light beam in the wavelength band λ1 (1260 nm to 1360 nm) and reflects a light beam in the wavelength band λ2α (1480 nm to 1560 nm) or higher, and when a multiplexed light beam is incident at an angle of 33 degrees, the filter film 51b reflects a light beam in the wavelength band λ2 or lower and passes a light beam in the wavelength band λ3 (1539 nm to 1620 nm) or higher.
When a multiplexed light beam (in the wavelength bands λ1, λ2 and λ3) is incident to the filter 51 of the wavelength splitting element 50 at an angle of 45 degrees to the normal line of the filter film 51b, the filter film 51b passes a light beam in the wavelength band λ1 and reflects a light beam in the wavelengths bands λ2 and λ3. The light beam (in the wavelength bands λ2 and λ3) reflected by the filter film 51b is incident to the filter 52 at an angle θ2 (6 degrees) relative to the normal line of the filter film 52. The filter film 52 passes the light beam in the wavelength band λ3 and reflects the light beam in the wavelength band λ2. The light beam reflected by the filter film 52b (in the wavelength band λ2) is incident to the filter film 51 at an angle θ3 (=45 degrees−2×θ2=33 degrees) and passes through the filter film 51. As described above, the wavelength splitting element 50 has a function of splitting a multiplexed light beam (in the wavelength bands λ1, λ2 and λ3) to light beams in the separate wavelength bands λ1, λ2 and λ3, and emitting the split light beams in separate directions a, b and c.
On the other hand, when light beams in the wavelength bands λ1, λ3 and λ2 are incident from the directions a, b and c, a multiplexed light beam (in the wavelength bands λ1, λ2 and λ3) can be acquired from a light path 53.
FIG. 11 shows a wavelength splitting element 54 with the same structure as that shown in FIG. 10, except the shape of the light transmission member. For example, glass is machined to a cube, and then the cube is split along the line f-f in FIG. 11 to form light transmission members 55a and 56a. A filter film 55b is provided on the face of one of the split faces, and the split faces are joined again to form a cube-shaped prism. A filter film 56b is formed on the face of the light transmission member 56a which is in the direction in which a light beam is reflected by the filter film 55b and from which a light beam is emitted. Next, an optical fiber held by a ferule is joined to an incident light path of light of the wavelength splitting element 54. As is the case with the filter film 51b and 52b as shown in FIG. 10, the filter films 55b and 56b are formed from a multilayer film made from SiO2, TiO2 or the like, and they have the same wavelength characteristics as the filter films 51b and 52b. The operation of the wavelength splitting element 54 is the same as that of the wavelength splitting element 50 shown in FIG. 10.
Meanwhile, JP-A-2000-143264 discloses a method for manufacturing for an optical device (beam splitter) by integrating two rectangular triangle prisms by joining slopes of the two prisms. FIG. 12 is a manufacturing process flow diagram of the beam splitter. FIGS. 13A, 13B and FIGS. 14A to 14F are process drawings for explaining the method for manufacturing of the beam splitter. A glass flat plate 60 shown in FIG. 13A has a structure in which a polarization split film 62 is formed on the top face and a matching film 63 on the bottom face of a glass plate 61. A plurality of glass flat plates 60 having exactly the same structure are used to form a laminated body. FIG. 13B shows a state in which the glass flat plates 60 are laminated at an angle of inclination of 45 degrees with use of a jig 64. The jig 64 is composed of a horizontal plate-shaped base 64a and an inclined side wall 64b which is fixed to the base 64a at an angle of inclination of 45 degrees relative to the base 64a. The glass flat plates 60 are sequentially laminated on the base 64a with the polarization split films 62 facing upward. At this time, one of the ends of each glass flat plate 60 is aligned along the inclined side wall 64b, and thereby the step-shaped laminated body 65 is formed in which the glass flat plates 60 are displaced with each other at an equal distance in the face direction. That is, a laminated body having a substantial parallelogram front view is formed. The glass flat plates 60 are joined to one another with an UV curing-type optical adhesive.
Next, as shown in FIG. 14A, the laminated body 65 that has been integrated by adhesive is cut along the dotted lines showing the angle of inclination of 45 degrees by a plurality of cutting planes 65a that are in parallel with each other at a given pitch using a wire saw or the like, and thereby split laminated bodies 66 that have been cut into pieces are obtained as shown in FIG. 14B. FIG. 14C shows the state in which the split laminated bodies 66 are placed horizontally. Top and bottom faces (i.e., cutting planes) of the split laminated bodies 66 are mirror polished and a reflective coat is coated on all faces. Portions with an acute angled protrusion at both ends of each split laminated body 66 may be cut. Alternatively, as shown in FIG. 14D, at first, the split laminated bodies 66 may be laminated in correct alignment to form the laminated body 67, and then the laminated body 27 may be cut after being tentatively adhered. The plurality of tentatively adhered split laminated bodies 66 are cut with the wire saw along the cutting planes 67a that are perpendicular to the above-described cutting planes 65a, and thereby a connected body 68 of the beam splitter shown in FIG. 14A is formed. Next, the cutting planes are mirror polished, and the beam splitter connected body 68 is cut along the dotted line 68a shown in FIG. 14E, and thereby a beam splitter 69 shown in FIG. 14F is obtained. This method for manufacturing is disclosed in JP-A-2000-143264.
However, the wavelength splitting element disclosed in JP-A-2005-164971 (FIG. 11, etc.) has caused poor isolation at individual emission ports. To improve isolation of the emission port by 30 dB or more by deposition of a multilayer film made from SiO2, TiO2 or the like, a multilayer film composed of approx. 60 to 100 layers need be formed, which causes poor yield. Further, internal stress of a thin film might cause coming off of the adhesion face and deterioration in reliability.
Further, although JP-A-2005-164971 discloses the structure of the wavelength splitting element, it does not disclose the method for manufacturing thereof.
Further, although JP-A-JP2000-143264 discloses the method for manufacturing of the beam splitter, it is difficult to conceive the method for manufacturing a wavelength splitting element even if the method is employed.